This invention relates generally to kaolin clays, and more specifically relates to a process for improving the rheological properties of kaolin clay slurries to render same more suitable for paper coating applications.
Most crude kaolinitic clays contain impurities which impair the properties of the clay for paper coating purposes; and among the most important of these are iron-containing compounds which are dark colored and reduce the overall brightness or reflectance to visible light, of the clay. It is known that the effect of certain of these discoloring impurities may be reduced by treating the clay with a reducing agent which converts ferric compounds to the soluble, less highly colored ferrous form. The most widely used reducing agents for these purposes are the hydrosulfites, such as sodium hydrosulfite.
In addition to the foregoing ferruginous impurities, many crude kaolin clays, including the sedimentary kaolins common to Georgia, include iron-stained titanium-based impurities such as iron-stained anatase and rutile. These titaniferous compounds may be at least partially removed from crude kaolins containing same, by subjecting the slurried crude to a froth flotation treatment. In a typical such sequence, and as is well-known in the art, the crude kaolin is formed into an aqueous slurry, the pH of the slurry is raised to an alkaline value, for example by addition of ammonium hydroxide, and a collecting agent is added, as for example, oleic acid. The slurry is then conditioned by agitating same for a relatively sustained period. A frothing agent, such as pine oil is then added to the conditioned slurry, after which air is passed through the slurry in a froth flotation cell to effect separation with the froth of substantial quantities of the titaniferous discolorants.
The classified underflow from the flotation cell is then commonly acidified, usually with sulfuric acid to a pH of about 3.0 to 5.0--as a prelude to the reductive bleaching step. The latter is then carried out using a hydrosulfite, such as the aforementioned sodium hydrosulfite.
At this point in the above conventional process, the slurry includes from about 15 to 30% solids, and prior to being subjected to a filtering step, alum may be added as a filtration aid. The filtration, e.g., by rotary vacuum filters dewaters the slurry to about 52 to 58% solids, after which the pH of the slurry is adjusted to about 7.0 with sodium hydroxide, and various dispersants are added to the slurry. The slurry can then be spray-dried and later reconstituted for use; or the slurry can be mixed with dried material to form a 70% solids slurry product--suitable, e.g., for shipping.
The various processing steps described above, unfortunately tend to introduce a variety of chemicals which can remain with the refined kaolin pigment and impair same in a number of respects, including by adversely affecting the rheological characteristics of the slurried kaolin products including the refined pigment. More specifically the large quantities of soluble sulfates introduced by oxidation of the hydrosulfites during bleaching, from the sulfuric acid added prior to bleaching, and from the alum, tends to increase the viscosity in slurried and dispersed kaolin products including the refined pigments. Thus, it may be found that a slurried kaolin product including such refined pigments, and intended for use in paper coating application, can display an undesirable "high-shear viscosity," where the quoted term refers to the coating clay characteristic defined in TAPPI Method T 648 su-72 (Rev. 1972).
It may further be noted that in the copending application of David G. Bell et al, Ser. No. 764,380, filed Jan. 31, 1977, now abandoned, a method is disclosed wherein refining of a kaolin crude is carried out while maintaining throughout a high solids content--i.e., usually between 60 and 75% solids by weight. No flotation step is used; rather a classification and a reductive bleaching step are sequentially conducted--all operations can be conducted at an alkaline pH to avoid flocculation, and no intermediate dewatering steps are used. In this high solids sequence, sulfate ion nonetheless is inevitably introduced via the reductive bleaching step, with consequent generation of viscosity problems in the refined product.